Our discovery of p59 (FKBP59) was made possible by development of cell line KN382/EC1, a mouse hybridoma secreting an IgG, directed against the nontransformed progestin receptor. It is present in rabbit uterus, kidney, and liver, calf uterus, Cos cells, human breast cancer cells, human lymphoma cells, and monkey uterus. Identification of p59 as FKBP59, an immunophilin, was accomplished by collaboration between our laboratory and that of Dr. Stewart Schreiber (see letter of collaboration). P59 (FKBP59) is an intracellular receptor for FK506 and rapamycin. We know that P59 is ubiquitous, associates with heat shock proteins, is in excess of, and a component of, nontransformed steroid receptors. It may be a heat shock protein. We believe p59 (FKBP59) is an important intermediate between the immunosuppressants and steroids, involved in protein trafficking and signal transduction. To prove or disprove this, we shall, 1. Determine intracellular targets for p59 (FKBP59). 2. Determine the binding and enzymatic characteristics of p59 (FKBP59). 3. Determine the endocrine (biological control) of p59 (FKBP59). We shall accomplish the first of our goals by means of KN382/EC1, FK506, and p59 affinity chromatography, coupled with one and two dimensional chromatography and protein sequencing of components of rabbit uterus and chick oviduct, including nontransformed progestin receptors. In addition, we will analyze cellular extracts of IM9, Jurkat, MCF-7 cells after hormonal and immunosuppressant treatment. Our second goal will be accomplished by binding assays (3H FK506, 3H ATP, etc.) of immunoprecipitated p59. Finally, after treating IM9, Jurkat, MCF, etc. cells with sex steroids, glucocorticoids, antihormones, and/or immunosuppressants we will study the cellular control of p59 by Western blotting, Northern blotting, and nuclear runoff transcription assays.